On Wed, Dec 02, 2009 at 08:04:26PM +0100, Jan Kara wrote: > > When using writev, the page we copy from is not paged in (while when we > > use ordinary write, it is paged in). This difference might be worth > > investigation on its own (as it is likely to heavily impact performance of > > writev) but is irrelevant for us now - we should handle this without data > > corruption anyway. > I've looked into why writev fails reliably the writes. The reason is that > iov_iter_fault_in_readable() faults in only the first IO buffer. Because > this is just 600 bytes big, following iov_iter_copy_from_user_atomic copies > only 600 bytes and block_write_end sets number of copied bytes to 0. Thus > we restart the write and do it one iov per iteration which succeeds. So > everything works as designed only it gets inefficient in this particular > case. Yep, this would be right. We could actually do more prefaulting; I think I was being a little over conservative and worried about earlier pages being unmapped before we were able to consume them... but I think being too worried about that case is optimizing an unusual case that is probably performing badly anyway at the expense of more common patterns. Anyway, what I was doing to test this code when I wrote it was to inject random failures into user copy functions. I guess this could be useful to merge in the error injection framework? Thanks, Nick -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
